(1) Field of the Invention
The present invention relates to a purification process for removing impurities such as alkali metal ions from valuable polyphenylene-sulfide.
(2) Description of the Related Art
Polyphenylene-sulfide (hereinafter referred to as "PPS") has an excellent heat resistance, strength, rigidity and other properties and is used as an engineering plastic material in various fields. In the electrical and electronic industries in particular, a large increase is expected in the demand for PPS as the material to be used for the production of IC encapsulation materials, laminated boards and the like, because PPS has excellent characteristics such as precision moldability, heat resistance and flame retardancy. Of course, in these fields, preferably the content of an electroconductive impurities such as an alkali metal compound is as low as possible. In PPS now marketed, the alkali metal ion content is 1,000 to 4,000 ppm, and accordingly, a reduction of the alkali metal ion content is desired.
The alkali metal ion referred to in the present invention means that derived from an alkali metal halide formed in the production of PPS and an unreacted alkali metal sulfide. These alkali metal compounds are combined intimately and tightly with PPS and are very difficult to remove them from PPS. Accordingly, various processes for removing alkali metal compounds from PPS have been proposed, as disclosed in Japanese Unexamined Patent Publications No. 57-108,135, 57-108,136 and 59-219,331. However, these processes are complicated and the removal effect is not satisfactory.
Namely, PPS once prepared is combined tightly and intimately with impurities such as alkali metal compounds and separation of the impurities is not easy. In the above-mentioned conventional processes for removing impurities from PPS, removal of impurities from PPS once prepared is intended, and therefore, the steps are inevitably complicated.